UV laser radiation is widely utilized now in variety of application fields including biology, chemistry, medicine, micromachining and photolithography. Some of these applications require very precise positioning of the micron-size focused laser beam on a micro-object. Today's commonly used scanning systems utilize electromechanical and mirror-based scanners, such as moving table and galvanometric scanning heads. Such device is described in U.S. Pat. No. 6,057,525 issued May 2, 2000. Here, a laser beam of visible wavelength is scanned by two—X and Y—mirrors angularly turned by electromechanical actuators. According to the patent, such scanning device provides accuracy of 1 micro-radian in scanning frequency range from 1 Hz to 1,000 Hz.
Such electromechanical scanners have very significant disadvantage, such as low scanning speed and other related to mechanical moving elements (inertia, low lifetime, etc.). Another, not-mechanical scanning method is described in U.S. Pat. No. 5,361,269 issued Nov. 1, 1994. Here, a variable-wavelength laser beam enters a diffraction grating. One-dimensional angular scanning is performed here by changing the laser wavelength that changes diffraction angle. According to the patent, such system provides single axis deflection only. Therefore, to perform two-axis deflection, this scanner has to have additional mechanical scanning device. Such solution, obviously, can not provide fast scanning speed too.
There is another solution, where scanning devices uses two-coordinate acousto-optical (AO) deflector working in UV waveband, such as one described in the article: “Anisotropic Bragg light deflector without midband degeneracy”, T. Yano et al. APL v.26, No 12, June 1975. The deflector comprises two TeO2 AO cells that work at 633-nm wavelength. RF signal feeding transducers of AO cells has the range of 38-88 MHz and provides deflection angle of 2.5 arc degrees.
There are some attempts to create AO deflector for UV radiation. To achieve deflection angle of 2 arc degrees these deflectors require higher frequency of RF signal, such as 140-240 MHz. Such high frequency, because of high-frequency acoustic signal absorption, causes overheating problems and requires special low-sickness transducer. Also, TeO2 crystal has to have trapezoidal shape, wherein the angle of input and output window inclination has to be 92.28 and 91.63 arc degrees respectively.
For example, Electro-Optical Products (EOPC), Inc. provides two-coordinate AO scanners DTXY-100, DTXY-250 and DTXY-400 based on TeO2 AO cells. These scanners work at 355-1,600-nm wavelength and provide 2.3-arc degree deflection at 1,064 nm. The efficiency of these deflectors does not exceed 50%.
Another UV scanner—UV266K—produced by EOPC comprises AO cells based on fused silica, which allows working with shorter wavelength (less than 180 nm), but drastically diminish angle of deflection (less than 0.4 arc degrees).
Therefore, higher deflection angles and shorter wavelength require high frequency and power of RF signal feeding the AO cell that cause distortion of deflected laser beam, overheating the crystal and, finally, causes the cell failure.
Thus, described above AO scanners do not allow completely utilizing all advantage of AO technology in UV waveband.
A laser scanner is commonly utilized as a core element of complex devices for scientific research and material processing, such as interactive microscopes and micromachining units. One of such application is a florescent analyzer that receives florescence induced by UV laser beam scanning a sample of biological or chemical substance. Another application of such device is an interactive microscope—the combination of optical microscope and laser scanner built in a single unit, wherein a sample that is visible in optical microscope can be controllably affected by focused laser beam precisely positioned on the sample by the laser scanner. Such device also allows controlling the processing in real time. Because the laser scanner is the main part of such devices, its characteristics determine the characteristics of whole device.